1. Field of the Invention
The invention relates to a device that determines whether or not an inter-cylinder air-fuel (A/F) ratio imbalance abnormality has occurred, and more particularly to a device that determines whether or not a relatively large imbalance of A/F ratio between the cylinders has occurred in a multi-cylinder internal combustion engine.
2. Description of Related Art
In an internal combustion engine provided with an exhaust gas control system using a catalyst, it is generally necessary to control a mixing ratio of air and fuel in a mixed gas burned in the internal combustion engine, that is, to control the A/F ratio in order to perform highly efficient catalytic purification of pollutants present in the exhaust gas. In order to perform such control of A/F ratio, an A/F ratio sensor is provided in an exhaust gas passage of the internal combustion engine and feedback control is performed such as to match the A/F ratio detected by the sensor with a predetermined target A/F ratio.
In a multi-cylinder internal combustion engine, the A/F ratio control is usually performed by using the same control amount for all of the cylinders. Therefore, the actual A/F ratio may vary between the cylinders even when the A/F ratio control is implemented. Where the degree of such imbalance is small, it can be absorbed by the A/F ratio feedback control and the processing of purifying pollutants contained in the exhaust gas can be performed with the catalyst. As a result, no effect is produced on exhaust gas emission and no problem is associated with such imbalance.
However, where the inter-cylinder imbalance of A/F ratio is large, for example, due to a failure of the fuel injection system of some cylinders, the exhaust gas emission is degraded, thereby causing problems. It is desirable that such a large imbalance of A/F ratio that causes degradation of exhaust gas emission be detected as an abnormality. In particular, in the case of internal combustion engines for automobiles, it is required that the inter-cylinder imbalance abnormality of A/F ratio be detected in the onboard state in order to prevent reliably the vehicle from running with degraded exhaust gas emission, and there is a recent trend to regulate such detection by law.
For example, in the device described in Japanese Patent Application Publication No. 2004-52620 (JP-A-2004-52620), the inter-cylinder imbalance value is calculated for each cylinder on the basis of variations in revolution speed of the internal combustion engine.
In the A/F ratio feedback control, a pre-catalyst sensor and a post-catalyst sensor may be provided as A/F ratio sensors upstream and downstream of the catalyst, respectively, the main A/F ratio control may be performed on the basis of the pre-catalyst sensor output and the auxiliary A/F ratio control may be performed on the basis of post-catalyst sensor.
Further, a prescribed guard range is typically provided in order to prevent the values of the control amount for the auxiliary A/F ratio control from being excessive.
Meanwhile, it is conceivable to detect the revolution speed variations in the internal combustion engine and to determine whether or not an inter-cylinder A/F ratio imbalance abnormality has occurred on the basis of the detected value.
The research results obtained by the inventor have demonstrated that in such a case there is a correlation between the auxiliary A/F ratio control amount and revolution speed variations, and where the absolute value of the auxiliary A/F ratio control amount increases to the vicinity of the guard value of the prescribed guard range, erroneous determination may occur when the imbalance abnormality is determined.